The invention relates to a household appliance comprising a drying chamber for drying wet articles therein, a process air loop for circulating process air to dry the articles and a heat pump, said heat pump comprising a heat transfer loop containing a heat transfer fluid to be circulated through said heat transfer loop, an evaporator heat exchanger for transferring heat from the process air into the heat transfer fluid by evaporating said heat transfer fluid, a liquefier heat exchanger for transferring heat from said heat transfer fluid to the process air by liquefying said heat transfer fluid, a compressor for compressing said heat transfer fluid and driving said heat transfer fluid through said heat transfer loop, and a nozzle for decompressing said heat transfer fluid.
A household appliance of this type is apparent from EP 0 467 188 B1. That document contains a detailed description of a household appliance that is configured as a dryer for drying articles which are wet laundry. The document refers to many details of the household appliance that may be necessary or at any rate advantageous in making or using the appliance. Accordingly, the whole content of this document is incorporated herein by reference.
Related art for household appliances is apparent from documents WO 2006/029953 A1 that specifies a dishwasher in relation to a laundry dryer or combined laundry washer and dryer, DE 197 38 735 C2 that discloses a household appliance with a different type of heat pump, EP 1 672 294 A2, and EP 1 672 295 A2, the latter two disclosing air conditioning devices that have cooling circuits which are in some aspects similar to the heat pump considered herein incorporated therein.
Drying of wet articles in a household appliance generally requires evaporating the humidity on the articles and transporting away by means of a current of heated process air. Such process air loaded with evaporated humidity may be discharged from the appliance, or subjected to a condensation process to recover the transported humidity in liquid form for collection and disposal. Such condensation process in turn required to cool the process air, thereby extracting heat. That heat may again be discharged from the appliance simply; in order to keep consumption of energy low however, it may be desired to recover that heat at least to an extent. To that end, a household appliance has been developed that incorporates a heat pump which recovers energy taken from the process air by evaporating a heat transfer fluid, subsequently compressing that heat transfer fluid and releasing heat from it back into the process air which circulates in an essentially closed loop. While it may be expedient or even required to open such process air loop at least occasionally as described in EP 0 467 188 B1, pertinent IEC standards require that a dryer that is claimed to recover humidity by condensation keeps any leakage of humidity below 20% of the total humidity present. Problems still to be encountered with such household appliances incorporating heat pumps are high manufacturing costs, relatively long periods needed to dry convenient charges of laundry or the like, and possible environmental hazards from heat transfer fluids applied in such appliances. To mitigate such hazards that are predominantly related to ozone-destroying properties of such compounds, chlorinated hydrocarbons that had been applied frequently in the past are presently prohibited from use due to pertinent legislation. Two other concerns that have grown to become determinative for the design of heat pump systems containing heat transfer fluids are the Global Warming Potentials of such compounds, that is their effect as infrared backscatterers when dispersed in the atmosphere, and, of course, their flammabilities.
The pickup of humidity from articles to be dried by process air is only effective if the process air is heated over any normal ambient temperature, preferably to a temperature higher than 60° C. That temperature will be brought down by the evaporation process to a somewhat lower temperature. At any rate, a temperature around or above 35° C. at an inlet of an evaporator heat exchanger may be expected to pose a problem to a heat pump of the type specified in the introductory chapter and designed in accordance with practice common in the art of refrigeration, in that compressors and refrigerant fluids (generally specified as “heat transfer fluids” herein) from normal refrigeration practice are not suitable for the purpose. It has been considered to obtain relief by reverting to refrigerants of remarkably high critical temperatures so as to ascertain their function at working temperatures up to 60° C., but no thorough analysis and guidance is available so far. Other measures that have been applied to obtain relief are bringing excess heat out of the appliance, by exhaling warm process air in exchange for cooler air and including additional heat exchangers to take excess heat from the heat transfer fluid. All of these measures, however, introduce further complexity and cost.
The known alkane R290 or propane has pertinent physical properties that make it highly suitable for the application considered herein, and it is noted that propane has already been used in commercially used refrigeration systems. In particular, propane has a Global Warming Potential index (“GWP index” in short hereinafter) of 3 that is remarkably low in comparison to a GWP index of 1300 for the conventional heat transfer fluid R134a. Of course, application of propane which is highly flammable will require dedicated protection of the system against fire hazard. Details of GWP indices of generally known refrigerant compounds are listed in the textbook “Solkane-Product Manual Refrigeration and Air-Conditioning Technology” by H. Buchwald, J. Hellmann, H. König, and C. Meurer, 2nd ed. 08/2000. As to a quantitative classification of refrigerants in view of their flammabilities as expressed in a Lower Flammability Level index, reference is made to European Standard document IEC 60335-2-40, “Household and Similar Electrical Appliances—Safety—particular Requirements for Electrical heat Pumps, Air-Conditioners and Dehumidifiers”, Edition 4.2 2005-07, Annex BB—Table BB.1. Pertinent information on refrigerants or heat transfer fluids is also available in U.S. Standard ASHRAE 34, including a specific nomenclature for such compounds and a classification on security and toxicity of such compounds.
Also, carbon dioxide or R744 is being considered for application in heat pump systems. While carbon dioxide is not flammable and has a GWP index as low as 1, carbon dioxide has a very low critical temperature that would not allow its application in a heat pump in a household appliance designed according to common practice.